1. Technical Field
The present disclosure relates to a communication apparatus and a communication method.
2. Description of the Related Art
IEEE 802.11 is one of the standards related to a wireless LAN and includes, for example, the IEEE 802.11ad standard (hereinafter referred to as “11ad standard”, see, for example, IEEE 802.11ad™-2012).
In the 11ad standard, a beamforming technique is used. Beamforming is a scheme for changing the directivity of at least one antenna of each of a transmitter and a receiver and setting the directivities of the antennas to optimize communication quality, for example, reception strength, to perform communication.
In the 11ad standard, a procedure called Sector Level Sweep (SLS) is defined to select, from among settings of directivities of a plurality of antennas (hereinafter referred to as “sectors”), an optimum sector. FIG. 1 is a diagram illustrating an overview of the procedure of SLS. SLS is performed between two terminals (hereinafter referred to as “STAs” standing for stations). One of the STAs is called “initiator” and the other is called “responder”.
First, the initiator changes sectors and transmits a plurality of Sector Sweep (SSW) frames. This transmission is called “Initiator Sector Sweep (ISS)”. In ISS, the responder measures the reception quality of each SSW frame.
Subsequently, the responder changes sectors and transmits a plurality of SSW frames. This transmission is called “Responder Sector Sweep (RSS)”. At this time, each SSW frame that is used in RSS is transmitted, with the SSW frame including information for specifying the SSW frame having the highest reception quality in ISS. In RSS, the initiator measures the reception quality of each SSW frame.
Finally, the initiator transmits an SSW Feedback (SSW-FB) frame that includes information for specifying the SSW frame having the highest reception quality in RSS. The responder may transmit SSW Acknowledgement (SSW-ACK) indicating that the responder has received the SSW-FB.
A description has been given above of SLS for performing beamforming training of transmission (Transmitter Sector Sweep, TXSS). Also, SLS may be used to perform beamform training of reception (Receiver Sector Sweep, RXSS). In this case, the STA that transmits SSW frames sequentially transmits the SSW frames in a single sector, whereas the STA that receives the SSW frames receives the SSW frames while switching the sector of a reception antenna for each SSW frame.
FIG. 2 is a diagram illustrating the configuration of an SSW frame. The SSW frame includes seven fields. A Frame Control field includes, for example, information representing the type of frame. A Duration field indicates a time until the current ISS or RSS is completed. RA indicates the MAC address of the STA that is to receive the SSW frame. TA indicates the MAC address of the STA that transmits the SSW frame. The length of the MAC address is 6 octets.
An SSW field includes five subfields. A Direction subfield whose value is 1 indicates that the SSW frame is transmitted by the initiator. A Direction subfield whose value is 0 indicates that the SSW frame is transmitted by the responder.
A CDOWN subfield indicates the value of a down counter indicating the number of remaining SSW frames to be transmitted in ISS or RSS. For example, when the value of the CDOWN subfield is 0, the SSW frame is the last SSW frame to be transmitted in ISS or RSS.
A Sector ID subfield indicates the ID of the sector that is used to transmit the SSW frame. A Directional Multi Gigabit (DMG) Antenna ID is an ID indicating the array antenna used for transmission when the transmitter includes a plurality of array antennas.
An RXSS Length subfield is used to give notice of the number of SSW frames that are necessary to perform RXSS by the STA that is performing transmission.
In the 11ad standard, the SSW frame including the above-described fields and subfields has a length of 26 octets.
As described above, in SLS in the 11ad standard, an SSW frame has a length of 26 octets. In each of ISS and RSS, SSW frames are transmitted the number of which is the same as the number of sectors where beamforming training is performed.